1. Field of the Invention
This invention relates to a magnetic toner used in a recording process such as electrophotography, electrostatic recording, electrostatic printing or toner jet recording.
2. Related Background Art
As methods for electrophotography, methods are conventionally known in which copied or printed images are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a toner to form a toner image, and transferring the toner image to a transfer medium such as paper as occasion calls, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor.
In a method of developing the electrostatic latent image, a magnetic one-component developing system making use of a magnetic toner is preferably used because it does not require any carrier and is advantageous for making apparatus compact. In the toner used in such a magnetic one-component developing system, a fine powdery magnetic material is mixedly dispersed in a considerable quantity, and the state of presence of this magnetic material has a great influence on the fluidity and triboelectric charging performance of the toner. Also, in the magnetic toner, the magnetic material is used as it is as a colorant in many cases. Accordingly, in order to improve coloring power of the toner, it is important for the magnetic material to be made small in particle diameter and have uniform dispersibility. However, when made small in particle diameter, the magnetic material comes apt to have a reddish tint and also comes very difficult to disperse uniformly in the toner. Hence, this tends to cause various problems.
For example, where toners are made to have small particle diameter in order to improve dot reproducibility in digital machines or the like, a toner containing the magnetic material in a large quantity may accumulate on a toner carrying member (developing sleeve) if the magnetic material stands dispersed insufficiently, image density may lower and the non-uniformity of tone that is called sleeve ghost may occur. Problems may also arise such that the magnetic material tends to come off because of rubbing friction between toner particles themselves or between toner particles and a toner layer thickness control member, and that the magnetic material having come off scratches the photosensitive member (latent image bearing member) to cause defective images.
To cope with this problem, proposals concerning magnetic iron oxides to be contained in magnetic toners have been made, but still there is room for improvement.
For example, Japanese Patent Application Laid-Open No. S62-279352 discloses a magnetic toner containing a magnetic iron oxide incorporated with silicon element. In such a magnetic iron oxide, the silicon element is intentionally brought into existence inside the magnetic iron oxide, but still there is room for improvement in the fluidity of the magnetic toner containing the magnetic iron oxide. Japanese Patent Publication No. H3-9045 discloses adding a silicate to control the particle shape of magnetic iron oxide to be spherical. In the magnetic iron oxide obtained by this method, the silicon element is rich distributed inside the magnetic iron oxide particles because of the use of the silicate for the controlling of particle shape and the silicon element is less present at the surfaces of the magnetic iron oxide particles, thus, because of a high smoothness magnetic iron oxide particles, the fluidity of the magnetic toner can be improved to a certain extent. However, the adhesion between the binder resin and the magnetic iron oxide which constitute the magnetic toner is insufficient.
Meanwhile, a binder resin constituting the toner may include styrene resins, polyester resins and epoxy resins. Polyester resins are preferably used from the viewpoint of low-temperature fixing performance. Also, recently, a method is studied in which two or more types of polyester resins having different softening points are used in the form of a mixture so that fixing temperature range can be broadened. The use of a plurality of resins in the magnetic toner in this way makes it severer to make the magnetic material disperse uniformly in the resins in the step of heat melt-kneading when the toner is produced.
To cope with such a problem, some studies are made in order to improve the dispersibility of the magnetic material. Japanese Patent Application Laid-open No. 2001-296689 discloses an attempt to improve dispersibility by using a specific polyester resin in combination with a magnetic material on the particle surfaces of which carbon black has been adsorbed.
However, this measure is still unsatisfactory when the toner is made small in particle diameter or when the magnetic material with small particle diameter is used.